OTTAWA — Some taxpayers may be filing tax returns using erroneous information supplied by the Canada Revenue Agency, the federal auditor general warned Tuesday after tabling an audit that found just getting through to the department's helplines is an even greater challenge than the government lets on.

Michael Ferguson's latest report to Parliament said callers all too often get a busy signal or a message to hang up and try back later when they try to contact the taxman by telephone — and when they do get through, they're not guaranteed of getting the right answers to their questions.

"When we called the call centres of the Canada Revenue Agency and we posed our questions, about 30% of the responses that we got back were not right," Ferguson told a news conference — a "very concerning" finding that could be causing problems for Canadians who file their own returns.

The Canada Revenue Agency headquarters in Ottawa is shown on November 4, 2011. THE CANADIAN PRESS/Sean Kilpatrick

"We just have to assume that if people are getting wrong answers, then sometimes they are filling out the tax return based on those wrong answers."

Ferguson couldn't say how many people might be affected — only that some surely have been.

"It's very much reasonable to extrapolate that that is going to have an impact on people," he said.

"If people aren't told the right date for when interest will be charged on how much they owe, they may make a payment later than they should and they might get charged interest that they weren't expecting."

Ferguson's auditors also found that more than half of the 53.5 million calls that came in to the call centre during the audit period had to be blocked, since the system lacked the capacity to handle them. That meant callers either heard a busy signal or a message asking that they try back later.

Auditor General Michael Ferguson arrives to hold a press conference at the National Press Theatre in Ottawa on Tuesday Nov. 21, 2017, regarding his 2017 Fall Report. THE CANADIAN PRESS/Sean Kilpatrick

Actual agents answered only 36% of all incoming calls — the rest of those that weren't blocked were directed to an automated self-serve system — and provided incorrect information to auditors about 30% of the time.

Those numbers fly in the face of the agency's own claims that 90% of callers are able to connect when they reach out for service either through the self-service centre or by speaking to an agent.

"When we called the call centres of the Canada Revenue Agency and we posed our questions, about 30% of the responses that we got back were not right." - Michael Ferguson, Auditor General of Canada

But that doesn't take into account the fact that, on average, a taxpayer has to call about four times in a week just to get through to the agency, or the fact that more than half of the calls are blocked outright due to volume, the report said.

"We found that the agency's numbers didn't account for the 29 million calls it blocked in a year — more than half of its total call volume," Ferguson said. "Those calls either get a busy signal, a message to visit the agency's website, or a message to call back later."

The Canada Revenue Agency website is photographed on March 13, 2017. THE CANADIAN PRESS/Graeme Roy

In its written response to the report, the government said it agrees there's a need to improve the accuracy of information provided to taxpayers.

To that end, the CRA said it will launch a new system early next year for training its call centre agents.

It also acknowledged that its current call centre technology is "outdated" and said an upgrade will be forthcoming.